Lasers based on optical fiber, whether in whole or in part, can provide more flexible, rugged and relatively simple sources of optical energy. For example, an optical fiber laser can have a smaller footprint, or can be more efficient, or can require less sophisticated cooling arrangements as compared to, for example, a gas based laser, particularly where gain media, whether amplifiers or oscillators, use fiber as the gain medium. Considerable technical effort and development has been focused on the production and use of nanosecond (“ns”) and femtosecond (“fs”) and laser pulses for materials processing, and particularly on fiber lasers for producing such pulses for materials processing.
Q-switched lasers, and in particular Q-switched fiber lasers, readily produce ns pulses with technology that is becoming relatively well understood and that can be relatively inexpensively manufactured so as to be reliable and dependable, even in a harsh production environment. However, the pulses can be longer than desired, and result in more energy than necessary being delivered to a work piece, which can cause an undesirable “heat affected zone” (HAZ) and damage.
Fs pulses can feature high peak power and a short time duration that can facilitate removing, such as by ablation, a material without creating as much, or any, HAZ that can damage the material. Fs pulses, however, can be considerably more difficult to produce and can require considerably more complex technology. For example, amplifying such fs pulses directly can induce undesirable nonlinear effects, particularly in a fiber amplifier where the peak power is high due to the short time duration of the pulse and where the optical intensity, which triggers the nonlinear effects, is also high due to the relatively small cross sectional area of the typical optical fiber. Certain nonlinear effects, such as Stimulated Brillouin Scattering (SBS) or Stimulated Raman Scattering (SRS), simply prevent providing higher output powers at the desired wavelengths. Accordingly, fs fiber lasers often use Chirped Pulse Amplification (CPA) to avoid triggering nonlinear effects such as SBS and SRS. In a typical CPA system a fs seed pulse is stretched via a pulse stretcher that provides linear chirp, and then the stretched, linearly chirped pulse, which has lowered peak power and optical intensity, is linearly amplified and then compressed using a linearly chirped pulse compressor to a fs pulse having high peak power. The compressed pulse can be substantially transform limited. The CPA technique works well, but its implementation can be technically complex and typically uses bulky, free space components (e.g., a bulk grating pair stretcher as well as a similar compressor). A fs CPA system can be large (several times larger than a ns system) as well as costly (e.g., more than 10 times the price of a ns system). Though less prevalent than CPA systems, it is also known to produce fs output pulses starting from picoseconds (e.g., 10 ps) seed pulses that are directly generated from a mode locked fiber laser and subsequently amplified by a fiber amplifier that adds spectrum via self phase modulation while producing an amplified pulse of nJ or greater pulse energy. A low spectral dispersion diffraction grating pair compressor (e.g., 0.5 ps/nm-1 ps/nm) compresses the amplified pulses to about 300 fs.
Picosecond (“ps”) pulsed laser systems are also known, and can provide for certain materials processing an attractive compromise between the too long ns pulses and the short, but often complicated to produce, fs pulses. Ps pulses can be directly produced by a ps seed oscillator, which can be a fiber laser, and amplified directly by fiber laser amplifiers to provide ps output pulses without resorting to a complicated CPA stretcher—compressor arrangement. This simplicity can be seen as a desirable feature in comparison to fs systems, and helps reduce the size, price or complexity of a ps laser system as opposed to a fs laser system.
Temporally shaped pulses can also be of interest, particularly in the case of ns pulses, where having too high a pulse power for too long can create heating issues, as indicated above. Temporally shaping ns pulses often involves control circuitry controlling an external modulator, which adds cost and complexity to the system.
Applicants, however, have found that there can be drawbacks associated with the direct production of ps output pulses, particularly when using lasers using optical fiber. Applicants are also aware that shaped ps pulses can be of interest and that drawbacks associated with certain methods of producing shaped pulses should be minimized or avoided. Accordingly, it is an object of the present disclosure to provide improved methods and apparatus for making and using ps pulses.
Other objects will be apparent from a study of the remainder of the present disclosure, including the drawings and claims.